Copper-covered steel foil, negative electrode, and battery

ABSTRACT

A negative electrode collector using a copper-covered steel foil for carrying a negative electrode active material for lithium ion secondary batteries has a steel sheet as the core material thereof and has, on both surfaces thereof, a copper covering layer having a mean thickness t Cu  of from 0.02 to 5.0 μm on each surface, and of which the total mean thickness, t, including the copper covering layer  7  is from 3 to 100 μm with t Cu /t of at most 0.3. The steel sheet can be common steel, austenitic stainless steel, or ferritic stainless steel. The copper covering layer can be a copper electroplating layer (including one rolled after plating). On the surface of the copper covering layer, for example, a carbon-based active material layer that has been densified through strong roll pressing is formed, and the copper-covered steel foil and the carbon-based active material layer constitute the negative electrode collector.

This application is a Divisional of U.S. Ser. No. 13/737,294 filed onJan. 9, 2013, which is a Continuation of PCT/JP2011/065707 filed on Jul.8, 2011, which is published as WO2012/005355 on Jan. 12, 2012.

TECHNICAL FIELD

The present invention relates to a copper-covered steel foil fornegative electrode collectors of lithium ion secondary batteries. Theinvention also relates to a negative electrode collector for lithium ionsecondary batteries, which carries an active material held on thesurface of the copper-covered steel foil, and its production method, andalso to a lithium ion secondary battery using the negative electrodecollector.

BACKGROUND ART

Recently, for environmental protection, development of new energyalternative to fossil fuel such as petroleum oil or the like andtechnical development for efficiently utilizing energy have becomepromoted. As part of these, solar power generation and wind powergeneration are being rapidly popularized. However, these powergeneration methods utilizing natural energy are susceptible to theweather, and, therefore, the output power is often unstable.Accordingly, in massive introduction of such new energy, electricalstorage technology for leveling output power and also electrical storagetechnology for effectively utilizing the electric power to be generatedin light-load time such as nighttime or the like are indispensable. As arelatively large-scale storage battery for use for storage of such newenergy, there are mentioned a sodium sulfur battery (NAS battery), aredox flow battery, a lead storage battery, etc., and these have beentried in verification test researches.

On the other hand, as storage batteries for mobile electronic appliancessuch as typically mobile phones and notebook-size personal computers, alithium ion secondary battery has become widely popularized. Asrelatively large sized storage batteries on a level usable as drivingpower sources for hybrid vehicles or electric vehicles, at present, anickel hydrogen secondary battery is the mainstream. In future, however,for meeting the needs of high-performance storage batteries, lithium ionsecondary batteries are expected to be further popularized also fordriving power sources for vehicles. Further in future, application oflithium ion secondary batteries would be taken into consideration alsofor storage of new energy. In view of these, recently, high-capacitylithium ion secondary batteries have become strongly desired.

CITATION LIST Patent References

Patent Reference 1: WO2002/093679

Patent Reference 2: Japanese Patent 3838878

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

A lithium ion secondary battery has a configuration in which a positiveelectrode collector of an aluminum foil and a negative electrodecollector of a copper foil are arranged in a lithium ion electrolyticsolution. Various types of configurations are known, of which thoseconstructed by laminating a positive electrode collector and a negativeelectrode collector followed by winding them into a columnar shape andthose constructed by alternately laminating tens sheets of positiveelectrode collectors and negative electrode collectors are generalconfigurations. As a small-capacity battery, there is known aconfiguration constructed by laminating one sheet of a positiveelectrode collector and one sheet of a negative electrode collector. Thesurface of the positive electrode collector and that of the negativeelectrode collector each carry a positive electrode active material anda negative electrode active material held thereon, respectively. Thecollectors of the two electrodes are separated from each other via aseparator such as a resin porous film or the like.

In this description, the configuration of the inside of the battery inwhich these two electrode collectors are laminated is referred to as“electrode laminate”. Of tabular (sheet-like) metal materials, inparticular, one having a thickness of at most 100 μm is referred to as“foil”.

The aluminum foil and the copper foil for use in a lithium ion secondarybattery have a low strength and are therefore readily deformed in theproduction line in which the foil is coated with an active material; andconsequently, for producing collectors having a high shape accuracy,high-level special control is needed. Under poor control, the foil stripmay be broken in the production line. Regarding battery products, inparticular, “laminate-type” lithium ion secondary batteries where thebattery contents are sealed up in a mode of laminated package areadvantageous for large-size products as excellent in heat radiationcharacteristics, but on the other hand, the disadvantage thereof is thatthe electrode laminate may be deformed when a local external force isapplied thereto from outside the batteries and the collector may bethereby readily damaged. Further, the volume of the active material maychange through charge-discharge cycles in use of battery products;however, it is difficult to completely uniformize the arrangement of theelectrode laminate inside the battery, and therefore the part where thegiven stress is concentrated may be readily damaged when the strength ofthe collector is low.

On the other hand, for providing high-capacity batteries, it is desiredto increase the discharge capacity per unit volume of the collectortherein. For this, it is advantageous that the active material exists ata high density on the surface of the collector. For increasing thedensity of the active material layer, it is effective to strongly pressthe coating film of the active material with a roll press or the like.However, as described below, it is difficult to further density theactive material layer of the collector that uses a conventional aluminumfoil or copper foil.

FIG. 1 schematically shows the condition of the cross section of amaterial in forming an active material layer according to aroll-pressing method. A coating film 2 that contains an active materialis formed on the surface of the metal foil 1 that is a core material ofthe collector; and by pressing by the rotating roll 3, the thickness ofthe coating film 2 is reduced to form the active material layer 4. Ingeneral, the metal foil 1 is an aluminum foil in a positive electrodecollector, and is a copper foil in a negative electrode collector. InFIG. 1, the metal foil 1, the coating film 2 and the active materiallayer 4 are overdrawn in point of the thickness thereof, and thethickness ratio does not always reflect the actual dimension.

FIG. 2 schematically shows the condition of the cross section of thematerial in passing through rolls, as seen in the direction A in FIG. 1,in which a suitable rolling force is given to the material in formingthe active material layer according to a roll-pressing method. In casewhere the rolling force given by the rolls 3 is a suitable one, theactive material layer 4 can be formed with little deformation of themetal foil 1. In FIG. 2, the metal foil 1 and the active material layer4 are overdrawn in point of the thickness thereof.

FIG. 3 schematically shows the condition of the cross section of thematerial in passing through rolls, as seen in the direction A in FIG. 1,in which an excessive rolling force is given to the material in formingthe active material layer according to a roll-pressing method. In thiscase, the rolling force given by the rolls 3 is larger than that in thecase of FIG. 2. With the increase in the given rolling force, the activematerial layer 4 may be more densified. However, the metal foil 1 is analuminum foil or a copper foil and the strength thereof is low, andconsequently, there has occurred plastic deformation in the center partin the width direction of the foil, therefore providing a so-called“center buckle” state. In case where an uncoated part 5 is provided ataround the edges of the metal foil 1 in the width direction thereof, thethickness difference between the edges and the center part may becomemore remarkable. The center buckle state brings about various problemsof shape failure and dimensional accuracy reduction with collectormaterials. Consequently, the rolling force by the rolls 3 is controlledto fall within a range within which the aluminum foil or the copper foildoes not deform, and this is a bar to the densification of the activematerial layer 4.

An object of the present invention is to provide a negative electrodecollector having higher strength and durability and to provide anegative electrode collector having a larger discharge capacity, as oneelemental technique that may bring about providing high-capacity lithiumion secondary batteries. Another object is to provide a lithium ionsecondary battery using the negative electrode collector.

Means for Solving the Problems

The above-mentioned objects can be attained by a copper-covered steelfoil for carrying a negative electrode active material for lithium ionsecondary batteries, which has a steel sheet as a core material thereofand has, on both surfaces thereof, a copper covering layer having a meanthickness t_(Cu) of from 0.02 to 5.0 μm on each surface, and of whichthe total mean thickness, t, including the copper covering layer is from3 to 100 μm with t_(Cu)/t of at most 0.3. The copper covering layerincludes, for example, a copper electroplating layer (including one thathas been rolled after plating) and a copper foil layer integrated withthe steel sheet through cladding.

For the steel sheet that is the core material of the copper-coveredsteel foil, usable here are a cold-rolled common steel sheet and anaustenitic or ferritic stainless steel sheet as the material thereof. Asstandardized products of a case of common steel, for example, those madeof materials of cold-rolled steel plates (including steel strips) asdefined in JIS G3141:2009 are applicable. To a case of stainless steel,for example, steel plates (including steel strips) having an austeniticor ferritic chemical composition as defined in JIS G4305:2005 areapplicable.

Concrete content ranges of the component elements that constitute thesteel sheet are exemplified below.

[Common Steel]

In terms of % by mass, C: 0.001 to 0.15%, Si: 0.001 to 0.1%, Mn: 0.005to 0.6%, P: 0.001 to 0.05%, S: 0.001 to 0.5%, Al: 0.001 to 0.5%, Ni:0.001 to 1.0%, Cr: 0.001 to 1.0%, Cu: 0 to 0.1%, Ti: 0 to 0.5%, Nb: 0 to0.5%, N: 0 to 0.05%, with balance of Fe and inevitable impurities.

[Austenitic Stainless Steel]

In terms of % by mass, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001to 2.5%, P: 0.001 to 0.045%, S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr:15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5%, Nb: 0 to 1.0%, Ti: 0 to1.0%, Al: 0 to 0.1%, N: 0 to 0.3%, B: 0 to 0.01%, V: 0 to 0.5%, W: 0 to0.3%, total of Ca, Mg, Y, REM (rare earth metal): 0 to 0.1%, withbalance of Fe and inevitable impurities.

[Ferritic Stainless Steel

In terms of % by mass, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001to 1.2%, P: 0.001 to 0.04%, S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5to 32.0%, Mo: 0 to 2.5%, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%,Al: 0 to 0.2%, N: 0 to 0.025%, B: 0 to 0.01%, V: 0 to 0.5%, W: 0 to0.3%, total of Ca, Mg, Y, REM (rare earth metal): 0 to 0.1%, withbalance of Fe and inevitable impurities.

In the above, the element of which the lowermost limit is 0% is anoptional element. The copper-covered steel foil that uses any of thesesteel sheets has a higher strength than that of the copper foilheretofore applied to ordinary collectors. In particular, thecopper-covered steel foil having a controlled tensile strength of from450 to 900 MPa is advantageous in enhancing the durability ofcollectors, and more advantageously, the tensile strength thereof iscontrolled to be more than 600 to 900 MPa.

The invention also provides a negative electrode collector for lithiumion secondary batteries, which is produced by forming a negativeelectrode active material layer for lithium ion secondary batteries, onthe surface of at least one copper covering layer of the above-mentionedcopper-covered steel foil. Further, the invention provides a lithium ionsecondary battery using the negative electrode collector for thenegative electrode thereof. Here “at least one copper covering layer”means one or both of the copper covering layers to cover the twosurfaces of the copper-covered steel foil. In case where a carbon-basedactive material is applied, preferably, the density of the activematerial layer is at least 1.50 g/cm³ like usual. For increasing thedischarge capacity, more preferably, the density of the carbon-basedactive material is at least 1.80 g/cm³, even more preferably at least2.00 g/cm³.

The invention also provides a method for producing a negative electrodecollector for lithium ion secondary batteries, which comprises:

a step of forming a coating film that contains a carbon-based activematerial for negative electrodes of lithium ion secondary batteries, onthe surface of at least one copper covering layer of the above-mentionedcopper-covered steel foil, and

a step of roll-pressing the coating film, after drying it, to reduce thecoating film thickness by from 30 to 70% thereby densifying the coatingfilm.

In this case, preferably, the density of the coating film is increasedto at least 1.80 g/cm³ by roll pressing, more preferably to at least2.00 g/cm³.

Advantage of the Invention

The invention has made it possible to provide a metal foil for negativeelectrode collectors for lithium ion secondary batteries that has ahigher strength than conventional one. Accordingly, the durability ofbatteries can be enhanced, and the invention can meet the needs ofincreasing the area of collectors and reducing the thickness thereof. Inaddition, the metal foil can be prevented from undergoing plasticdeformation to be caused by volume change of the negative electrodeactive material in charge/discharge of batteries, and is thereforeadvantageous for life prolongation of batteries. Further, in theproduction step for negative electrode collectors, the metal foil hardlydeforms, and therefore collectors having a higher dimensional accuracycan be realized. In particular, it is easy to much more densify theactive material layer than usual, and a negative electrode collectorhaving a high discharge capacity can be produced at a low cost.Accordingly, the invention contributes toward improving the durabilityof lithium ion secondary batteries, prolonging the life thereof andincreasing the capacity thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A view schematically showing the condition of the cross sectionof a material in forming an active material layer on the surface of ametal foil according to a roll-pressing method in a collector productionstep for lithium ion secondary batteries.

FIG. 2 A view schematically showing the condition of the cross sectionof the material in passing through rolls, as seen in the direction A inFIG. 1, in which a suitable rolling force is given to the material informing the active material layer according to a roll-pressing method.

FIG. 3 A view schematically showing the condition of the cross sectionof the material in passing through rolls, as seen in the direction A inFIG. 1, in which an excessive rolling force is given to the material informing the active material layer according to a roll-pressing method.

FIG. 4 A view schematically showing the cross-sectional configuration ofthe copper-covered steel foil of the invention for carrying a negativeelectrode active material thereon.

FIG. 5 A view schematically showing the cross-sectional configuration ofthe negative electrode collector of the invention.

MODE FOR CARRYING OUT THE INVENTION

FIG. 4 schematically shows the cross-sectional configuration of thecopper-covered steel foil of the invention for carrying thereon anegative electrode active material for lithium ion secondary batteries.Both surfaces of the steel sheet 6 are covered with the copper coveringlayer 7 to constitute the copper-covered steel foil 10. In FIG. 4, thethickness of the copper covering layer 7 is overdrawn (the same shallapply to FIG. 5 to be mentioned below). On both surfaces, the coppercovering layer 7 is so controlled that the mean thickness t_(Cu) fallsfrom 0.02 to 5.0 μm on each surface, and the total mean thickness, t,including the copper covering layer falls from 3 to 100 μm. The ratio oft_(Cu)/t is at most 0.3 on the two surfaces. Preferably, the coppercovering layer 7 has nearly a uniform thickness on both surfaces. Thecopper covering layer 7 may be formed, for example, according to acopper electroplating method to be mentioned below. In case where thesteel sheet 6 is stainless steel, preferably, a nickel strike platinglayer is formed as the metal underlayer. So far as the mean thicknesst_(Cu) of the copper covering layer 7 and the mean thickness t of thecopper-covered steel foil 10 each fall within the range satisfying theabove-mentioned condition, the presence between the steel sheet 6 andthe copper covering layer 7 of one or more metal underlayers having goodadhesiveness to the two layers causes no problem. However, the totalmean thickness t_(M) on each surface of the other metal underlayers thancopper is preferably so controlled that the total (t_(M)+t_(Cu)) withthe mean thickness t_(Cu) of the copper covering layer 7 formed on theunderlayers is at most 5.0 μm. In case where copper strike plating isgiven, it is considered that the copper strike plating layer constitutesa part of the copper covering layer 7.

When the mean thickness t of the copper-covered steel foil 10 is lessthan 3 μm, then it would be difficult to make the copper-covered steelfoil 10 fully secure the strength and the necessary active materialcarrying amount for collectors, even when a high-strength steel sheet 6is applied thereto. The thickness may be controlled to fall within arange of at least 5 μm, or at least 7 μm. On the other hand, when t ismore than 100 μm, then it could not meet the requirement for small-sizedlarge-capacity batteries. In general, it is desirable that the thicknessis controlled to fall within a range of at most 50 μm, or may becontrolled to fall within a range of at most 25 mμ, or at most 15 μm.

When the mean thickness t_(Cu) on each surface of the copper coveringlayer 7 is less than 0.02 μm, then the absolute amount ofhigh-conductivity copper in the copper-covered steel foil 10 may reduceand defects such as pinholes and the like running through the coppercovering layer 7 may increase, whereby it would be difficult to maintaina high and stable discharge capacity of batteries. t_(Cu) may becontrolled to be at least 0.03 μm, or at least 0.05 μm. On the otherhand, when t_(Cu) is more than 5.0 μm, then the copper covering layer 7would readily undergo plastic deformation in case where the rollingforce is increased in the roll-pressing step, and it would be difficultto realize a more highly densified active material layer whilemaintaining a high dimensional accuracy thereof. Another disadvantage isthat the copper plating cost would increase. In case where emphasis isplaced on securing the dimensional accuracy of the collector and ondensifying the active material layer, more preferably, t_(Cu) iscontrolled to fall within a range of at most 1.0 μm or less than 1.0 μm.

When the mean thickness t of the copper-covered steel foil 10 is reducedto at most 20 μm or so, if the mean thickness t_(Cu) of the coppercovering layer 7 is not reduced with that, then it would be difficult toprevent the copper covering layer 7 from undergoing plastic deformationin roll pressing. As a result of various investigations, when t_(Cu)/tis at most 0.3, the deformation of the copper covering layer 7 can beeffectively restrained by the steel sheet 6 and the range isadvantageous for producing collectors with high dimensional accuracy.More preferably, t_(Cu)/t is at most 0.2, or even at most 0.1.

The copper-covered steel foil 10 using the steel sheet 6 as the corematerial thereof has a dramatically higher strength as compared with thecopper foil used in conventional collectors. Use of the steel sheet 6controlled to have the above-mentioned component composition is moreeffective. For stably and remarkably enhancing the durability of thecollector when it is incorporated in a battery and enhancing the shaperetention capability (center buckling preventing ability) in a rollpressing step for forming the negative electrode active material layer,it is more effective to make the copper-covered steel foil 10 have atensile strength of from 450 to 900 MPa. The tensile strength may becontrolled to be at least 500 MPa. In particular, the copper-coveredsteel foil 10 that is controlled to have a strength on a level more than600 MPa or on a level more than 650 MPa is extremely advantageous forenhancing the reliability of collectors. The tensile strength of thecopper-covered steel foil 10 can be controlled by selecting the chemicalcomposition of the steel sheet 6 and by controlling the cold-rollingreduction ratio before obtaining the final copper-covered steel foil 10.Even when the tensile strength is so increased as to be more than 900MPa, any more enhancement of durability and shape retention capabilitycould not be expected but on the contrary, there may increase somedisadvantage of cost increase owing to the increase in the cold-rollingreduction ratio.

FIG. 5 schematically shows the cross-sectional configuration of thenegative electrode collector of the invention for lithium ion secondarybatteries. On the surface of the copper covering layer 7 thatconstitutes the copper-covered steel foil 10, formed is a negativeelectrode active material layer 40 that has been densified by rollpressing or the like. This drawing illustrates a case where the negativeelectrode active material layer 40 is formed on both surfaces of thecopper-covered steel foil 10; however, a negative electrode collectorwhere the negative electrode active material layer 40 is formed on onesurface alone may also be employable. For example, in the collector tobe positioned at the edges of an electrode laminate, the negativeelectrode active material layer 40 may be formed on one surface alone.

The mean thickness of the densified, negative electrode active materiallayer 40 is preferably from 5 to 150 μm on one surface, more preferablyfrom 20 to 100 μm. In a case of the negative electrode active materiallayer 40 that contains a carbon-based active material (to be mentionedbelow), preferably, the density thereof (including fine pores inside thelayer) is at least 1.50 g/cm³. According to the process to be mentionedbelow, the density of the carbon-based active material layer can be atleast 1.80 g/cm³, or even at least 2.00 g/cm³. The densified negativeelectrode active material layer provides not only the durabilityenhancing effect through strengthening of the metal foil 1 but also thedischarge capacity increasing effect per unit volume of the activematerial layer, over conventional negative electrode collectors (inwhich, for example, the density of the active material layer is from1.50 to 1.75 g/cm³ or so). On the other hand, however, when the densityof the active material layer is too high, an electrolytic solution couldhardly penetrate into the layer thereby providing a factor ofinterfering with charge movement. In consideration of the fact that thetheoretical density of graphite is 2.26 g/cm³, the density of thecarbon-based active material layer is preferably within a range of atmost 2.20 g/cm³, and may be controlled to fall within a range of at most2.15 g/cm³. The density of the active material layer can be calculatedfrom the mean thickness of the active material layer, which isdetermined through microscopic observation of the cross section ofcollector, and the mean mass per unit area of the active material layer.

[Exemplification of Production Process of Negative Electrode Collector]

Examples of a production process for producing the copper-covered steelfoil of the invention and then using it for producing a collector forlithium ion secondary batteries, include the following A to D. Insidethe parenthesis [ ], an intermediate or final material is shown.

A. →[cold-rolled steel plate]→rolling into foil→copperplating→[copper-covered steel foil]→coating with activematerial-containing coating material→drying of coating film→rollpressing→shaping and working by cutting, etc.→[negative electrodecollector]

A2. →[cold-rolled steel plate]→rolling into foil→copper plating→furtherrolling→[copper-covered steel foil]→coating with activematerial-containing coating material→drying of coating film→rollpressing→shaping and working by cutting, etc.→[negative electrodecollector]

B. →[cold-rolled steel plate]→copper plating→rolling intofoil→[copper-covered steel foil]→coating with active material-containingcoating material→drying of coating film→roll pressing→shaping andworking by cutting, etc.→[negative electrode collector]

C. →[cold-rolled steel plate]→cladding with copper foil→rolling intofoil→[copper-covered steel foil]→coating with active material-containingcoating material→drying of coating film→roll pressing→shaping andworking by cutting, etc.→[negative electrode collector]

D. →[cold-rolled steel plate]→rolling into foil→cladding with copperfoil→[copper-covered steel foil]→coating with active material-containingcoating material→drying of coating film→roll pressing→shaping andworking by cutting, etc.→[negative electrode collector]

In the above-mentioned process A, a cold-rolled steel plate is, afterrolled into a foil having a predetermined thickness, plated with copperin the step of producing the copper-covered steel foil; in the processA2, a cold-rolled steel plate is, after rolled into a foil, plated withcopper, and is further rolled into the copper-covered steel foil havinga predetermined thickness; in the process B, a cold-rolled steel plateis, after plated with copper, rolled into a foil to be a copper-coveredsteel foil having a predetermined thickness. As a the strike plating,employable here is copper strike plating or nickel strike plating. Inthe process C, a cold-rolled steel plate is clad with a copper foil, andthen further rolled into a copper-covered steel foil having apredetermined thickness; and in the process D, a cold-rolled steel plateis rolled into a foil, and then clad with a copper foil to be acopper-covered steel foil having a predetermined thickness.

[Steel Sheet]

As the steel sheet that is the core material of the copper-covered steelfoil of the invention, employable are common steel and also stainlesssteel. Stainless steel is excellent in corrosion resistance and isfavorable for use where emphasis is placed on securing high durabilityand reliability. The concrete chemical composition range is as describedabove.

[Copper Plating]

As a method for forming a copper covering layer, employable here is acopper plating method as exemplified in the above-mentioned processes A,A2 and B. In the invention, employable is any known copper platingtechnique, for example, including electroplating, chemical plating,vapor-phase plating, etc. As chemical plating, there may be mentionedelectroless plating; and as vapor-phase plating, there may be mentionedsputtering, ion plating. Of those, the copper electroplating method isfavorable for mass-production, since the plating layer can be formedrelatively rapidly and economically and since the plating thickness iseasy to control.

Copper Electroplating:

Employable here are various known copper electroplating methods. Thecopper electroplating condition in one case of using a sulfuric acidbath is exemplified. For example, using a plating bath that containscopper sulfate of from 200 to 250 g/L and sulfuric acid of from 30 to 75g/L and has a liquid temperature of from 20 to 50° C., the cathodecurrent density may be from 1 to 20 A/dm². However, depending on a caseof rolling a copper-plated foil to have a predetermined thickness aftercopper plating or a case of directly forming a copper covering layerhaving an intended thickness though copper plating, the copper platingamount varies greatly. In the former case, it is necessary to form thecopper plating layer of which the thickness is estimated through backcalculation from the intended thickness of the copper covering layer, inaccordance with the rolling reduction ratio in the later step. When thenecessary copper plating thickness could not be obtained in the firsttry in the copper plating line, the plate may be led to pass through thecopper plating line several times.

Pretreatment for Copper Electroplating:

In copper electroplating, a pretreatment of nickel strike plating may beperformed. In particular, when the steel sheet is stainless steel,nickel strike plating is especially effective for enhancing theadhesiveness of the copper plating to the steel sheet. The nickel strikeplating condition may be set as follows. For example, using a platingbath at normal temperature that contains nickel chloride of from 230 to250 g/L and hydrochloric acid of 125 ml/L and has a pH of from 1 to 1.5,the cathode current density may be from 1 to 10 A/dm².

In place of nickel strike plating, a pretreatment of copper strikeplating may be performed prior to copper electroplating. The copperstrike plating condition may be set as follows. For example, using aplating bath that contains copper pyrophosphate of from 65 to 105 g/L,potassium pyrophosphate of from 240 to 450 g/L in a ratio (P ratio) ofthe total pyrophosphate ion concentration (g/L) to the total copper ionconcentration (g/L) of from 6.4 to 8.0, and aqueous ammonia of from 1 to6 mL/L, and has a liquid temperature of from 50 to 60° C. and a pH offrom 8.2 to 9.2, the cathode current density may be within a range offrom 1 to 7 A/dm².

Vapor-Phase Plating:

The copper covering layer may be formed according to any knownvapor-phase plating method of vapor deposition, sputtering, ion platingor the like. A case of the production method through sputtering isexemplified. First, a cold-rolled common steel plate or a cold-rolledstainless steel plate is cold-rolled to a foil having a predeterminedthickness, using a foil-rolling machine, thereby giving a steel foil.The steel foil is pre-treated for degreasing in a wet washing line thatcomprises processing steps of “methylene chloridewashing→drying→isopropyl alcohol washing→water washing→drying”. Next,the degreased steel foil is led to pass through a continuous sputteringline to form a copper covering layer thereon. The continuous sputteringline may be constituted by, for example, a series of a coil withdrawingunit, a high frequency magnetron sputtering unit and a winding unitarranged in a vacuum chamber.

Concretely, for example, the sputtering may be performed according tothe following method. The argon partial pressure inside the chamber iscontrolled at around 0.1 Pa, and the surface of the steel foil isactivated through reverse sputtering at an output power of about 100 W.Next, a copper covering layer having a mean thickness t_(Cu) of about0.05 μm is formed on one surface of the steel foil through filmformation sputtering using pure copper as the target at an output powerof about 300 W. In this stage, the plate traveling speed in thecontinuous sputtering line is controlled to thereby adjust the intendedmean thickness t_(Cu). The operation is repeated while the surface andthe back of the steel foil are turned over, thereby giving acopper-covered steel foil using the steel sheet as a core material andhaving the copper covering layer on both surfaces thereof.

[Cladding]

As another method for producing the copper-covered steel foil,employable here is a method of cladding both surfaces of a cold-rolledsteel plate or steel foil with a copper foil. As the cladding method,known are a hot-cladding method, a cold-cladding method, an explosivebonding method, etc. In particular, the cold-cladding method is suitablefor mass-production since it is excellent in thickness accuracy andsecures good producibility.

Of the above-mentioned production process C, a method of using a coldcladding mode to produce a copper-covered steel foil is exemplified. Asthe materials, one cold-rolled steel strip and two copper foil strips ofwhich thicknesses have been so controlled that the above-mentioned ratiot_(Cu)/t could be a predetermined value are prepared. As the copper foilstrip, there are mentioned foil strips of tough pitch copper,oxygen-free copper, alloy copper, etc. These are led to pass through adegreasing line to remove the rolling oil therefrom, and then bothsurfaces of the cold-rolled steel strip are sandwiched between thecopper foil strips to give a three-layered laminate, which is thencontinuously cold-rolled for cold cladding, thereby producing a cladmaterial where the three layers have been integrated through cladding.When the cold-rolling reduction ratio in cladding is too low, then theappearance of a new plane at the interface between the steel strip andthe copper foil strip would be insufficient and the cladding strengthmay be thereby insufficient. When the cold-rolling reduction ratio istoo high, the rolling load may be excessive thereby providing someproblems in that the rolled shape may deform or, the rolled laminate maybe broken in the line owing to the excessive tensile load given thereto.As a result of various investigations, the cold-rolling reduction ratiofor cladding could fall within a range of from approximately 10 to 75%;however, the ratio is more preferably from 40 to 50% for a cold-rolledsteel plate of common steel, and from 15 to 40% for stainless steel. Theobtained clad material is cold-rolled with a foil-rolling machine togive a copper-covered steel foil.

The case is more concretely exemplified. For example, as a cold-rolledsteel plate, one steel strip having a thickness of 0.684 mm is prepared;and as copper foils, two copper foil strips each having a thickness of0.018 mm are prepared. These are layered to form a three-layer laminatematerial having a total thickness of 0.018+0.684+0.018=0.720 mm. This iscold-clad at a rolling reduction ratio of 50% to give a three-layer cladmaterial having a thickness of 0.36 mm. This is further led to passthrough a foil-rolling machine several times to give a copper-coveredsteel foil. In every rolling, the layer thickness ratio of the materialbefore cladding is kept nearly as such, and in this case, the coppercovering layer on both surfaces has t_(Cu)/t=0.018 mm/0.720 mm=0.025.When the mean thickness t of the obtained copper-covered steel foil is20 μm, the mean thickness t_(Cu) of the copper covering layer on onesurface is 0.025×20 μm=0.5 μm on both sides of the copper-covered steelfoil.

As in the above-mentioned production process D, a cold-rolled steelplate is previously rolled to a foil to give a steel foil, and the steelfoil may be cold-clad with a copper foil to give a copper-covered steelfoil. In this case, a copper-covered steel foil controlled to have apredetermined thickness can be directly produced using a claddingmachine; however, in this, the copper foil to be supplied forcold-cladding is extremely thin and may need special care in handlingit.

In the cold-cladding method, for realizing more stable and bettercladding performance, it is effective to perform cold-cladding operationin a non-oxidative atmosphere, a reduced-pressure atmosphere or a vacuumatmosphere. As pretreatment for cladding, it may also be effective topreviously activate the surface to be clad, through vapor-phase etchingsuch as argon plasma etching or the like.

[Rolling into Foil]

For rolling into foil in the above-mentioned production processes A toD, usable is any ordinary rolling machine capable of giving a highrolling force, such as a Sendzmir rolling machine, a cluster rollingmachine, etc. In these rolling machines, the work rolls are preventedfrom being elastically deformed by the action of many backup rolls, andtherefore the shape of the copper-covered steel foil or the steel foilto be produced is easy to control suitably. The rolling reduction ratior is represented by the following formula, in which the thickness beforerolling is t_(in) and the thickness after rolling is t_(out).Rolling Reduction Ratio r (%)(1−t _(out) /t _(in))×100

As described above, a steel sheet is used as the core material in thecopper-covered steel foil of the invention, and therefore, as comparedwith that of conventional copper foil for collectors, the strength levelof the copper-covered steel foil of the invention is high. Foroptimizing the strength level in accordance with the specifications ofbatteries, it is effective to suitably control the total rollingreduction ratio in cold rolling (including cold rolling in cladding)which the finally-annealed steel material receives until it becomes thecore material for the final copper-covered steel foil. As a result ofvarious investigations, for obtaining a copper-covered steel foil havingan especially high-level strength, it is extremely effective to attainthe above-mentioned total rolling reduction ratio of at least 90%; andfor further increasing the strength, the total rolling reduction ratiomay be 95% or even more. The uppermost limit of the total rollingreduction ratio is restrained mainly by the capability of the rollingmachine to be used, but any excessive strengthening would beuneconomical. In general, the total rolling reduction ratio may be goodto be at most 99%, and may be within a range of at most 98% inconsideration of the economic potential and producibility.

[Formation of Active Material-Containing Coating Film]

The negative electrode collector of the invention comprises thecopper-covered steel foil obtained in the above, and a negativeelectrode active material layer formed on the surface thereof. Thenegative electrode active material layer has pores into which anelectrolytic solution can penetrate to enable charge movementtherethrough with lithium ions, and contains a negative electrode activematerial, a conductivity additive, a binder, etc. The negative electrodeactive material may be any one that enables insertion and release oflithium ions. For example, there is mentioned a carbon-based activematerial that includes pyrocarbons, cokes (pitch coke, needle coke,petroleum coke, etc.), graphites, glassy carbons, fired organic polymers(prepared by firing and carbonizing furan resin or the like at asuitable temperature), carbon fibers, active carbons, etc. As theconductivity additive, usable here are, for example, graphites,acetylene black, ketjen black, channel black, furnace black, lamp black,thermal black, carbon fibers, metal fibers, etc. As the binder, usablehere are, for example, polyvinylidene fluoride (PVDF),polytetrafluoroethylene (PTFE), polyethylene, polypropylene,tetrafluoroethylene-hexafluoropropylene copolymer (FEP), vinylidenefluoride-hexafluoropropylene copolymer, etc.

In this description, the negative electrode active material using theabove-mentioned carbon material is referred to as “carbon-based activematerial”. The negative electrode active material layer using thecarbon-based active material is referred to as “carbon-based activematerial layer”. The process for forming the carbon-based activematerial layer comprises, for example as described above, processingsteps of “coating with active material-containing coatingmaterial→drying of coating film→roll pressing” may be adopted. In thiscase, first, a coating material that contains the carbon-based activematerial for negative electrodes of lithium ion secondary batteries asdescribed above (active material-containing coating material) isprepared, and this is applied onto the surface of the copper coveringlayer of a copper-covered steel foil according to a blade coater coatingmethod or the like. Subsequently, the coating film is dried. Thethickness of the dried coating film is estimated through backcalculation from the intended thickness of the active material layer,taking the coating film thickness reduction in the subsequentroll-pressing step, into consideration. In case where the activematerial layer is formed on both surfaces of the copper-covered steelfoil, preferably, the coating thickness is nearly uniform on the twosurfaces.

[Densification of Active Material Layer]

For increasing the discharge capacity of electrodes, it is effective toincrease the density of the active material layer. As a method forincreasing the density of the active material layer, generally employedis a method of reducing the thickness of the above-mentioned dry coatingfilm through roll pressing. In the invention, the copper-covered steelfoil having a high strength is used, and therefore, even when therolling force in roll pressing is increased, the metal foil hardlyundergoes plastic deformation. Accordingly, in the invention, therolling force in roll pressing can be increased more than before.

Concretely, it is desirable that the thickness of the dry coating filmis reduced by at least 30% through roll pressing to thereby increase thedensity of the resulting layer. The coating film thickness reductionratio is defined by the following formula (1):[Coating Film Thickness Reduction Ratio (%)]=(h ₀ −h ₁)/h ₀×100   (1)

wherein h₀ means the mean coating film thickness (μm) on one surfacebefore roll pressing; and h₁ means the mean coating film thickness (μm)after rolling press. In case where emphasis is placed on densifying theactive material layer, the coating film thickness reduction ratio ismore effectively at least 35%, even more preferably at least 40%.However, when the rolling force is too large, then the coating filmdensity would be excessive, and if so, electrolytic solution could notpenetrate into the coating film easily and a space necessary for chargemovement through the layer could not be fully secured. Anotherdisadvantage is that the metal foil would be ununiformly deformed. As aresult of various investigations, the coating film thickness reductionratio by roll pressing is preferably within a range of at most 70%, andmay be controlled to be at most 60%.

[Lithium Ion Secondary Battery]

The negative electrode collector that has the negative electrode activematerial layer that has been densified in the manner as above, on thesurface of the above-mentioned copper-covered steel foil may be combinedwith a positive electrode collector via a separator to give an“electrode laminate”, and as combined with an electrolytic solution,this constitutes a lithium ion secondary battery. For the positiveelectrode collector, the separator and the electrolytic solution, anyknown materials used for lithium ion secondary batteries and any othernew materials usable in place of them are employable here.

The electrolytic solution is exemplified. As the solvent, for example,there are mentioned nonaqueous solvents such as ethylene carbonate (EC),diethyl carbonate (DEC), propylene carbonate, butylene carbonate,dimethyl carbonate, sulfolane, dimethoxyethane, tetrahydrofuran,dioxolan, etc. One or more of these may be used either singly or ascombined. As the solute, for example, there are mentioned LiClO₄, LiPF₆,LiBF₄, LiAsF₆, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂),LiC(CF₃SO₂)₃, LiCF₃(CF₂)₃SO₃, etc. One or more of these may be usedeither singly or as combined.

As the shape of the casing material to seal up and case therein theabove-mentioned electrode laminate and the electrolytic solution, thereare mentioned coin shape, cylindrical shape, rectangular shape, laminatesheet pack-type or the like. As the material of the casings, there arementioned aluminum or its alloy, titanium or its alloy, nickel or itsalloy, copper or its alloy, stainless steel, common steel, nickel-platedsteel sheet, copper-plated steel sheet, zinc-plated steel sheet, etc.For the laminate sheet pack-type casing, for example, usable is alaminate foil prepared by laminating a heat-sealable resin film on ametal foil such as an aluminum foil, stainless steel foil, etc.

EXAMPLES

Various types of copper-covered steel foils were produced according tothe above-mentioned production processes A to D, and tested forcorrosion resistance thereof in an electrolytic solution. Somecopper-covered steel foils were tested to measure the tensile strengththereof. In addition, using the respective copper-covered steel foils,negative electrode collectors for lithium ion secondary batteries wereproduced, and tested to evaluate the discharge capacity thereof.Examples codes (a, b, etc.) correspond to the codes of “productionprocess” described in Table 2 given hereinunder.

[Production of Copper-Covered Steel Foil Having Common Steel Sheet asCore Material]

Example a

This example demonstrates production of a copper covered steel foilaccording to the above-mentioned production process B.

A plurality of cold-rolled steel strips (annealed strips) each havingthe following chemical composition and having a thickness of 0.3 mm anda width of 200 mm were prepared.

Chemical Composition: in terms of % by mass, C: 0.003%, Al: 0.038%, Si:0.003%, Mn: 0.12%, P: 0.012%, S: 0.122%, Ni: 0.02%, Cr: 0.02%, Cu:0.01%, Ti: 0.073%, N: 0.0023%, with balance of Fe and inevitableimpurities.

In a continuous electroplating line, both surfaces of the steel stripwere processed for copper strike plating and copper electroplating (mainplating) to give copper-plated steel strips having a copper platinglayer in a varying thickness. In one copper-plated steel strip, thecopper plating layer thickness on both surfaces was nearly uniform.Subsequently, this was cold-rolled with a foil-rolling machine, therebygiving a copper-covered steel foil of which the mean thickness tincluding the copper plating layer on both surfaces was 20 μm and ofwhich the mean thickness t_(Cu) on one surface of the copper coveringlayer was in a different stage of from 0.9 to 0.005 μm. In this case,the thickness of the copper plating layer (that is, the total platingamount of the copper strike plating and the main plating) to be formedon the surface of the steel strip having a thickness of 0.3 mm wasestimated through back calculation in such a manner that, when the meanthickness t of the copper-covered steel foil was controlled to be 20 μmafter cold rolling, the mean thickness t_(Cu) of the copper coveringlayer could be a predetermined one falling within a range of from 0.9 to0.005 μm. For example, in case where a copper-covered steel foil havingt_(Cu) of 0.5 μm is to be obtained, the plating thickness shall be 7.9μm on one surface.

For the copper strike plating, used was a copper strike plating bathcontaining copper pyrophosphate of 80 g/L, potassium pyrophosphate of300 g/L and aqueous ammonia of 3 mL/L, and having a P ratio of 7, aliquid temperature of 55° C. and a pH of 9. The cathode current densitywas 5 A/dm², and the plating thickness on one surface was 0.3 μm.

For the copper electroplating (main plating), used was a copper platingbath containing copper sulfate of 210 g/L and sulfuric acid of 45 g/Land having a liquid temperature of 40° C. The cathode current densitywas 10 A/dm².

For confirming the formation of the copper covering layer having thepredetermined thickness, a copper-covered steel foil that had been soproduced as to have the copper covering layer having an intendedthickness of 0.5 μm was cut and polished in a mode of ion-milling, thenobserved with an electron microscope, and the thickness of the coppercovering layer was measured. From one sample, three test pieces werecollected at three sites spaced from each other at intervals of 5 m inthe rolling lengthwise direction, and three views of each one test piecewere observed. As a result, 3 views×3 sites=9 views in total wereanalyzed. The minimum value of the found data was 0.42 μm, the maximumvalue thereof was 0.55 μm, and the mean value thereof was 0.48 μm.Accordingly, it was confirmed that the foil rolling that had beencarried out herein realized nearly the just intended accurate rolling.

[Production of Copper-Covered Steel Foil Having Stainless Steel Sheet asCore Material]

Example b

This example demonstrates production of a copper-covered steel foilaccording to the above-mentioned production process A.

Commercially-available, cold-rolled steel strips of SUS 304 and SUS 430(both annealed strips corresponding to JIS G4305:2005) were cold-rolledwith a foil-rolling machine, thereby preparing steel foils each having athickness of 20 μm. In an electroplating line, both surfaces of thesteel foil were processed for nickel strike plating and copperelectroplating to produce a copper-covered steel foil having a meanthickness t_(Cu) of the copper covering layer on one surface of 0.5 μmor 0.05 μm. The nickel strike plating thickness was about 0.2 μm on onesurface. In one copper-covered steel foil, the copper covering layer hadnearly a uniform thickness on both surfaces of the foil.

Example c

This example demonstrates production of a copper-covered steel foilaccording to the above-mentioned production process A2.

A commercially-available, cold-rolled steel strip of SUS 304 (annealedstrip corresponding to JIS G4305:2005) was cold-rolled with afoil-rolling machine, thereby preparing a steel foil having a thicknessof 20 pin. In an electroplating line, both surfaces of the steel foilwere processed for nickel strike plating and copper electroplating toproduce a copper-covered steel foil (intermediate product) having a meanthickness of the copper covering layer on one surface of 0.5 μm. Thenickel strike plating thickness was about 0.2 μm on one surface. Thecopper-covered steel foil was further rolled with a foil-rolling machineto produce a copper-covered steel foil having a mean thickness tincluding the copper covering layer of 8.0 μm and having a meanthickness t_(Cu) of the copper plating layer on one surface of 0.2 μm.The copper covering layer thickness on both surfaces was uniform.

Example d

This example demonstrates production of a copper-covered steel foilaccording to the above-mentioned production process C.

A cold-rolled, SUS 430-equivalent ferritic stainless steel strip(annealed strip) having the following chemical composition and having athickness of 0.684 mm and a width of 300 mm was prepared.

Chemical Composition: in terms of % by mass, C: 0.058%, Al: 0.009%, Si:0.56%, Mn: 0.31%, P: 0.021%, S: 0.005%, Ni: 0.20%, Cr: 16.7%, Mo: 0.32%,Cu: 0.031%, N: 0.030%, with balance of Fe and inevitable impurities.

Two rolled copper foil strips each having the following chemicalcomposition and having a thickness of 18 μm and a width of 300 mm wereprepared.

Chemical Composition: in terms of % by mass, O: 0.0003%, P: 0.0002%,with balance of Cu and inevitable impurities.

The above-mentioned cold-rolled stainless steel strip and rolled copperfoil strips were led to pass through a degreasing washing line tothereby remove the rolling oil therefrom respectively, and thenconfigured in three layers in such a manner that the cold-rolledstainless steel strip could be sandwiched between the rolled copper foilstrips each put on the surface and the back thereof, and thereafter ledto pass through a continuous cold-cladding line. This was thuscold-rolled to a rolling reduction ratio of 50%, thereby giving athree-layer clad structure having a thickness of 0.36 mm. This wasfurther cold-rolled with a foil-rolling machine, thereby giving acopper-covered steel foil, of which the mean thickness t including thecopper covering layer was 20 μm and of which the mean thickness t_(Cu)on one surface of the copper covering layer was 0.5 μm.

Example e

This is another example that demonstrates production of a copper-coveredsteel foil according to the above-mentioned production process C.

A cold-rolled, SUS 430-equivalent ferritic stainless steel strip androlled copper foil strips each having the same composition as in theabove-mentioned Example d were prepared. The cold-rolled stainless steelstrip had a thickness of 1.8 mm and a width of 300 mm; and the rolledcopper foil strips each had a thickness of 38 μm and a width of 300 mm.According to the same process as in the above-mentioned Example d, athree-layer clad structure was produced at a cold-rolling reductionratio of 50%. This was further cold-rolled with a foil-rolling machine,thereby giving a copper-covered steel foil, of which the mean thicknesst including the copper covering layer was 100 μm and of which the meanthickness t_(Cu) on one surface of the copper covering layer was 2.0 μm.

Example f

This is still another example that demonstrates production of acopper-covered steel foil according to the above-mentioned productionprocess C.

A cold-rolled, SUS 430-equivalent ferritic stainless steel strip androlled copper foil strips each having the same composition as in theabove-mentioned Example d were prepared. The cold-rolled stainless steelstrip had a thickness of 0.5 mm and a width of 300 mm; and the rolledcopper foil strips each had a thickness of 63 μm and a width of 300 mm.According to the same process as in the above-mentioned Example d, athree-layer clad structure was produced at a cold-rolling reductionratio of 50%. This was further cold-rolled with a foil-rolling machine,thereby giving a copper-covered steel foil, of which the mean thicknesst including the copper covering layer was 50 μm and of which the meanthickness t_(Cu) on one surface of the copper covering layer was 5 μm.

Example g

This example demonstrates production of a copper-covered steel foilaccording to the above-mentioned production process D.

A SUS 430-equivalent ferritic stainless steel strip (annealed strip) androlled copper foil strips each having the same composition as in theabove-mentioned Example d were prepared. The cold-rolled stainless steelstrip had a thickness of 0.6845 mm and a width of 300 mm; and the rolledcopper foil strips each had a thickness of 12 μm and a width of 300 mm.The cold-rolled stainless steel strip was cold-rolled with afoil-rolling machine to give a steel foil strip having a thickness of 15μm. The steel foil strip was kept sandwiched between the rolled copperfoil strips on both surfaces thereof, and rolled in a continuouscold-cladding line at a cold-rolling ratio of 38%, thereby giving acopper-covered steel foil, of which the mean thickness t including thecopper covering layer was 15 μm and of which the mean thickness t_(Cu)on one surface of the copper covering layer was 4.5 μm.

[Corrosion Resistance Test in Electrolytic Solution]

The above-mentioned copper-covered steel foils having any steel sheet ofcommon steel, SUS304 or SUS430 as the core material thereof were testedfor the corrosion resistance thereof in an electrolytic solution. Testpieces having a size of 30×50 mm, as cut out of each copper-coveredsteel foil, were tested. At the edges of the test piece, the steel baseof the steel sheet was exposed out. As an electrolytic solution forlithium ion secondary batteries, a solution was prepared in which LiPF₆was dissolved, at a concentration of 1 mol/L, in a mixed solvent ofethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio by volumeof 1/1. A globe box equipped with a vapor circulating purifier was usedhere. In the globe box in which the oxygen concentration and themoisture concentration were controlled to be at most 1 ppm each, thetest piece was immersed in the electrolytic solution at 25° C. for 4weeks. The corrosion resistance was evaluated through measurement of themass of the test piece before and after the immersion test, and throughICP-AES quantitative analysis of Fe and Cu dissolved in the electrolyticsolution.

As a result, any significant mass change was not detected in every testpiece before and after the immersion test. The Fe concentration and theCu concentration in the electrolytic solution in which each of the testpieces had been immersed were each less than the detection limit (lessthan 1 ppm) in ICP-AES analysis, and therefore could not be quantified.No dissolution of Fe and Cu in the electrolytic solution was detected.From these, it has been confirmed that the above-mentioned respectivecopper-covered steel foils show good corrosion resistance in theelectrolytic solution for lithium ion secondary batteries.

[Tensile Test]

The above-mentioned copper-covered steel foils produced through thecopper plating step (the foils of the invention with t_(Cu)=0.5 μm,produced in Examples a and b), a commercial copper foil (comparativefoil), and a commercial aluminum foil (comparative foil) were tested ina tensile test using a universal precision tensile tester. The dimensionof the test piece was 12.7 mm in width and 175 mm in length; and therolling direction was the lengthwise direction thereof. In the initialstage, the chuck-to-chuck distance was 125 mm, and the tensile test wasconducted at a pulling rate of 2 mm/min until each test piece broke. Inthe test, the maximum load given to the test piece was divided by theinitial cross section (measured value) of the test piece to give thetensile strength of the tested foil. Every sample was tested formultiple test runs of n=3. The found data were averaged and the averagevalue is referred to as the tensile strength of the tested foil. Theresults are shown in Table 1.

TABLE 1 Copper covering layer Thick- ness Thick- on one Tensile nessSteel Forming surface Strength Group Type (μm) Sheet Method (μm) (MPa)Foil of the copper- 20.0 common copper 0.5 710 Invention covered steelplating steel foil Foil of the copper- 21.4 SUS430 copper 0.5 755Invention covered plating steel foil Foil of the copper- 21.4 SUS304copper 0.5 854 Invention covered plating steel foil Comparative copper20   — — — 210 Foil foil Comparative copper 18   — — — 420 Foil foil(hard) Comparative aluminum 20   — — — 87 Foil foil

It is found that the copper-covered steel foils of the invention have anextremely high strength, as compared with the copper foil used for thenegative electrode collector in existing lithium ion secondary batteriesand with the aluminum foil used for the positive electrode collectortherein. The strength of the copper-covered steel foil can be controlledon a different level by controlling the cold-rolling reduction ratio inthe production process. The tensile strength of the copper-covered steelfoils shown in Table 1 is a case of some examples, and the presentinventors have separately confirmed that the tensile strength of thecopper-covered steel foils of the invention can be controlled within arange of from 450 to 900 MPa. According to the investigations made bythe present inventors, it is quite possible to produce, by the use ofexisting rolling technology, copper-covered steel foils having a tensilestrength of more than 600 MPa or even more than 650 MPa when varioustypes of steel are used for steel sheets.

[Production of Negative Electrode Collector Sample]

90 parts by mass of graphite powder as a negative electrode activematerial, 5 parts by mass of acetylene black as a conductivity additive,and 5 parts by mass of polyvinylidene fluoride as a binder were mixed,and the resulting mixture was dispersed in N-methyl-2-pyrrolidone forpreparing a slurry, and thereby obtaining an active material-containingcoating material. The coating material was applied on one surface of thecopper-covered steel foil produced in the Example and on one surface ofa copper foil having a thickness of 20 μm, thereby forming thereon acarbon-based active material-containing coating film. The coating filmwas dried, and then roll-pressed for increasing the density of theactive material layer, thereby forming a carbon-based active materiallayer. Accordingly, a negative electrode collector sample was thusproduced. For the roll pressing, two conditions were employed here. Theload per unit length in the roll axis direction (sample width direction)to be given by the roll to the sample (hereinafter referred to as“linear pressure”) was 1 tonf/cm (980 kN/m) in one condition, and was 2tonf/cm (1960 kN/m) in the other condition. In this, the negativeelectrode active material layer was formed on one surface alone of themetal foil to produce the negative electrode collector sample; however,even in the case where the active material layer is formed on bothsurfaces, the influence of the linear pressure on the density of theactive material layer is basically the same as that in the case wherethe layer is formed on one surface alone. The linear pressure and thecoating film thickness reduction ratio, as defined by theabove-mentioned formula (1), are shown in Table 2.

[Measurement of Density of Active Material Layer]

A cross section of the negative electrode collector sample was polishedin a mode of ion-milling, then the cross section was observed with anoptical microscope equipped with a CCD camera. On the digital image ofthe cross section texture taken by the CCD camera, the thickness of thecarbon-based active material layer was measured. In one sample, threeviews were observed, and the mean thickness of the active material layerwas calculated. A disc sample having a diameter of 35 mm was blanked outof the negative electrode collector sample, and the mass of the discsample was measured. Next, the disc sample was immersed in anN-methyl-2-pyrrolidone solution for 1 week so that the carbon-basedactive material layer on the surface of the sample was completely peeledoff. The mass of the peeled test sample was measured. Using the massdifference before and after peeling the layer, and the found data of themean thickness of the active material layer, the density of the activematerial layer was obtained. The results are shown in Table 2.

[Evaluation of Discharge Capacity]

A disc piece having a diameter of 15.958 mm (area of 2 cm²) was blankedout of the negative electrode collector sample and used as a test piecefor discharge capacity measurement. A globe box equipped with a vaporcirculating purifier was used here. In the globe box in which the oxygenconcentration and the moisture concentration were controlled to be atmost 1 ppm each, an ordinary three-electrode test cell having a workingelectrode, a reference electrode and a counter electrode wasconstructed. As the test cell housing, used here was Hohsen Corp.'sHS-3E. In this, the above-mentioned test piece for discharge capacitymeasurement was set as a work electrode, and a metal lithium foil wasused as the reference electrode and the counter electrode. As theseparator to partition between the work electrode and the referenceelectrode and that to partition between the counter electrode and thereference electrode, used was a polypropylene-made porous film(thickness 25 μm). As the electrolytic solution, used was a solutionprepared by dissolving LiPF₆ at a concentration of 1 mol/L in a mixedsolvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in aratio by volume of 1/1.

Of each test cell, the oretical capacity that the active material haswas calculated. Next, using the current value expressed by [theoreticalcapacity (mAh)]/5(h), the cell was completely charged, and thendischarged at the same current value. The discharge capacity in thisstage is referred to as [battery capacity (mAh)] of each test cell.Subsequently, this was completely charged at a constant charging rate of0.5 CmA, and then discharged at a constant discharging rate of 1.0 CmA.This is one cycle, and each test cell was repeatedly tested for 10cycles. On the 10th cycle, the discharge capacity Q₁₀ per unit volume ofthe active material layer was measured. The test temperature was 25° C.In this, the charging rate and the discharging rate are expressed by thefollowing formulae (2) and (3), respectively.[Charging Rate (CmA)]=[Battery Capacity (mAh)]/[Charging Time (h)]  (2)[Discharging Rate (CmA)]=[Battery Capacity (mAh)]/[Discharging Time(h)]  (3)

The discharge capacity was evaluated using a collector sample with acopper foil as the metal foil (No. 12 in Table 2) as the standardsample, and the discharge capacity ratio defined by the followingformula (4):[Discharge Capacity Ratio]=[Q ₁₀ of Test Sample to be evaluated]/[Q ₁₀of Standard Sample]  (4)

The results are shown in Table 2.

TABLE 2 Metal Foil Negative electrode Collector Copper Sample after rollpressing covering Active Copper layer Roll Pressing Material coveringMetal Thick- Coating Layer Active layer Foil ness Film Mean MaterialDis- Forma- Produc- Thick- (on one Linear thickness thick- Layer chargeSteel tion tion ness t surface) Pressure Reduction ness Density CapacityGroup No. Type Sheet Method Process (μm) t_(Cu) (μm) t_(Cu)/t (tonf/cm)Ratio (%) Shape (μm) (g/cm³) Ratio Example  1 copper- common copper a20.0 0.9  0.0450 2 41 good 38 2.14 1.13 of the covered steel platingInvention steel foil Example  2 copper- common copper a 20.0 0.5  0.02502 41 good 39 2.14 1.14 of the covered steel plating Invention steel foilExample  3 copper- common copper a 20.0 0.1  0.0050 2 41 good 38 2.151.15 of the covered steel plating Invention steel foil Example  4copper- common copper a 20.0 0.05  0.0025 2 41 good 39 2.13 1.14 of thecovered steel plating Invention steel foil Example  5 copper- commoncopper a 20.0 0.02  0.0010 2 41 good 38 2.15 1.15 of the covered steelplating Invention steel foil Compar-  6 copper- common copper a 20.00.005 0.0003 2 41 good 38 2.14 0.18 ative covered steel plating Examplesteel foil Example  7 copper- common copper a 20.0 0.5  0.0250 1 32 good45 1.84 1.01 of the covered steel plating Invention steel foil Example 8 copper- SUS304 copper b 21.4 0.5  0.0234 2 41 good 38 2.14 1.13 ofthe covered plating Invention steel foil Example  9 copper- SUS304copper b 20.5 0.05  0.0024 2 41 good 39 2.13 1.14 of the covered platingInvention steel foil Example 10 copper- SUS430 copper b 21.4 0.5  0.02342 41 good 38 2.15 1.15 of the covered plating Invention steel foilExample 11 copper - SUS430 copper b 20.5 0.05 0.0024 2 41 good 38 2.141.14 of the covered plating Invention steel foil Compar- 12 copper — — —20.0 — — 1 31 center 45 1.82 1.00 ative foil buckled Example Compar- 13copper — — — 20.0 — — 2 — broke — — — ative foil Example Example 14copper- SUS430 cladding d 20.0 0.5  0.0250 2 41 good 38 2.14 1.15 of thecovered Invention steel foil Example 15 copper- SUS430 cladding e 1002.0  0.0200 2 40 good 39 2.10 1.16 of the covered Invention steel foilExample 16 copper- SUS430 cladding g 15 4.5  0.3000 2 40 good 39 2.081.15 of the covered Invention steel foil Example 17 copper- SUS304copper c 8.0 0.2  0.0250 2 41 good 38 2.13 1.09 of the covered platingInvention steel foil Example 18 copper- SUS430 cladding f 50 5.0  0.10002 40 good 39 2.09 1.15 of the covered Invention steel foil

In Comparative Example No. 12 in Table 2, a copper foil that hasheretofore been used was used as a metal foil, and the densification ofthe active material layer was tried by strong roll pressing to such adegree as to provide center buckling (standard sample); and as comparedwith that in a negative electrode collector in conventional ordinarylithium ion secondary batteries, the active material layer density inthis case was increased. In Comparative Example No. 13, the same copperfoil as in No. 12 was used as the metal foil but further stronger rollpressing was tried therein. In this, therefore, the sample broke in theroll pressing step since the strength of the copper foil is low.

In Examples of the invention in Table 2, the copper-covered steel foilhaving, on the surfaces thereof, a copper covering layer with athickness of at least 0.02 μm was used, and roll-pressed on the samelevel as or on a higher level than that in Comparative Example No. 12(standard sample) to thereby density the active material layer therein.In these Examples, the collectors produced all had a good shape. Aboveall, in Examples where the samples were roll-pressed strongly to such adegree that copper foil would be broken (Nos. 1 to 5, 8 to 11, 14 to18), the density of the active material layer was much more increased,and with that, the discharge capacity increased noticeably.

On the other hand, among the copper-covered steel foils, that inComparative Example No. 6 was poor in the discharge capacity since thethickness t_(Cu) of the copper covering layer was too small.

DESCRIPTION OF REFERENCE NUMERALS

-   1 Metal Foil-   2 Coating Film-   3 Roll-   4 Active Material Layer-   5 Uncoated Part-   6 Steel Sheet-   7 Copper covering layer-   10 Copper-covered Steel Foil-   40 Densified Negative Electrode Active Material Layer

The invention claimed is:
 1. A copper-covered steel foil for carrying anegative electrode active material for a lithium ion secondary battery,which has a steel sheet as a core material thereof and has, on bothsurfaces thereof, a copper covering layer having a mean thickness t_(Cu)of from 0.02 to 1.0 μm on each surface, and of which the total meanthickness, t, including the copper covering layer is from 3 to 100 μmwith t_(Cu)/t of at most 0.3.
 2. The copper-covered steel foil asclaimed in claim 1, wherein the copper cove ng layer is a copperelectroplating layer (including one roped after plating).
 3. Thecopper-covered steel foil as claimed in claim 1, wherein the steel sheetis loaned of a cold-rolled steel plate (including steel strip) asdefined in JIS G3141:2009.
 4. The copper-covered steel foil as claimedin claim 1, wherein the steel sheet has an austenitic or ferriticchemical composition as defined in JIS G4305:2005.
 5. The copper-coveredsteel foil as claimed in claim 1, wherein the steel sheet has acomposition comprising, in terms of % by mass, C: 0.001 to 0.15%, Si:0.001 to 0.1%, Mn: 0.005 to 0.6%, P: 0.001 to 0.05%, S: 0.001 to 0.5%,Al: 0.001 to 0.5%, Ni: 0.001 to 1.0%, Cr: 0.001 to 1.0%, Cu: 0 to 0.1%,Ti: 0 to 0.5%, Nb: 0 to 0.5%, N: 0 to 0.05%, with a balance of Fe and aninevitable impurity.
 6. The copper-covered steel foil as claimed inclaim 1, wherein the steel sheet has a composition comprising, in termsof % by mass, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%,P: 0.001 to 0.045%, S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to26.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0to 0.1%, N: 0 to 0.3%, B: 0 to 0.01%, V: 0 to 0.5%, W: 0 to 0.3%, totalof Ca, Mg, Y, REM (rare earth metal): 0 to 0.1% with a balance of Fe andan inevitable impurity.
 7. The copper-covered steel foil as claimed inclaim 1, wherein the steel sheet has a composition comprising, in termsof % by mass, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%,P: 0.001 to 0.04%, S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5 to 32.0%,Mo: 0 to 2.5%, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to0.2%, N: 0 to 0.025%, B: 0 to 0.01%, V: 0 to 0.5%, W: 0 to 0.3%, totalof Ca, Mg, Y, REM (rare earth metal): 0 to 0.1%, with a balance of Feand an inevitable impurity.
 8. The copper-covered steel foil as claimedin claim 1, which has a tensile strength of from 450 to 900 MPa.
 9. Thecopper-covered steel foil as claimed in claim 1, which has a tensilestrength of from more than 600 to 900 MPa.
 10. A negative electrode fora lithium ion secondary battery, which has an active material layer forthe negative electrode of a lithium ion secondary battery, as formed onthe surface of at least one copper covering layer of the copper-coveredsteel foil of claim
 1. 11. A negative electrode for a lithium ionsecondary battery, which has a carbon-based active material layer forthe negative electrode of a lithium ion secondary battery, as formed onthe surface of at least one copper covering layer of the copper-coveredsteel foil of claim 1, at, a density of at least 1.50 g/cm³.
 12. Anegative electrode for a lithium ion secondary battery, which has acarbon-based active material layer for the negative electrode of alithium ion secondary battery, as formed on the surface of at least onecopper covering layer of the copper-covered steel foil of claim 1, at adensity of at least 1.80 g/cm³.
 13. A negative electrode for a lithiumion secondary battery, which has a carbon-based active material layerfor the negative electrode of a lithium ion secondary battery as formedon the surface of at least one copper covering layer of thecopper-covered steel foil of claim 1, at a density of at least 2.00g/cm³.
 14. A lithium ion secondary battery using the negative electrodeof claim 10 for a negative electrode therein.